A decanter centrifuge generally includes a rotating bowl, typically having a cylindrical portion and a frusto-conical end portion. The rotation of the bowl creates a centrifugal force which separates a liquid feed mixture into its constituent parts. The feed mixture within the bowl forms a cylindrical pond, with a ring or layer of separated heavy material adjacent the inside of the bowl wall and a ring or layer of lighter material radially inward of the heavy material layer.
The terms "heavy phase" and "light phase" are employed hereinafter to describe materials which are separable from the feed mixture by the decanter centrifuge through the application of centrifugal force. The light phase material will usually be a liquid and the heavy phase material will usually be a mixture of solids and liquid. The liquid feed mixture introduced into the bowl generally has a specific concentration of suspended solids or other insoluble material therein. These "solids" are generally concentrated by the centrifugal force to form a heavy phase or mixture of varying concentration within the rotating bowl, including coarse solids, fine solids and liquid. Because of the varying degrees in density of the solids as well as the varying degrees of centrifugal force acting on those solids within the bowl, the concentration of the separated heavy phase may vary within the bowl. The concentration of the heavy materials that do not settle from the liquid material also varies.
A screw conveyor, the distinguishing feature of a decanter centrifuge, rotates inside the bowl at a slightly different speed from the bowl. The flights of the screw conveyor push the separated heavy phase along the inside of the bowl wall towards the conical end of the bowl. Discharge ports for the separated heavy phase are located at the small diameter of the conical bowl portion. The separated light phase liquid is discharged by flowing from the cylindrical pond through separate discharge ports. The light phase liquid discharge ports are located, typically, at the opposite end of the bowl from the heavy phase discharge ports.
Separation of the heavy phase materials from the feed mixture is a function of the residence time of the mixture in the bowl, a function of the feed rate, and the ability of the centrifuge to separately discharge the heavy and light phase materials. The purpose of the decanter centrifuge is to separately discharge a concentrated heavy phase and a clarified liquid. In order for the heavy phase to be discharged, it must be moved up the incline of the conical end portion of the bowl, called the beach, against the centrifugal force component acting in the opposite direction downward along the beach.
The separate discharge of heavy phase and light phase material from a decanter centrifuge has been the subject of a number of patents for decanter centrifuges. Typically, a decanter centrifuge operates with the heavy phase discharge port being radially inward with respect to the weir surface of the light phase discharge ports. This operation, known as a "positive dam" or "below spillover", requires that the heavy phase material be moved by the conveyor across a portion of the beach where there is no overlying liquid layer.
Ambler U.S. Pat. No. 3,172,851 describes the operation of a decanter centrifuge with the liquid discharge weirs set at a "negative dam" or "above spillover" position, i.e., at a position radially-inward of the weir surface of the heavy phase discharge ports. The Ambler-type operation takes advantage of the force of the liquid on the heavy phase along the entire length of the beach to help the conveyor move heavy phase material up the beach toward the heavy phase discharge ports. The relative radial difference between the weir surfaces is intended to be slight. The Ambler-type operation relies on the cohesive nature of the heavy phase material to form a dam that prevents the liquid head (the height of the liquid layer radially inward of or above the heavy phase discharge weir surface) from washing over the heavy phase weir surface.
Within the Ambler-type operation, the heavy phase layer in the conical end of the bowl is totally immersed in the liquid until the moment before discharge. Therefore, the heavy phase will be relatively wet. (In a "below spillover" type operation, the heavy phase emerges from the liquid on the beach and is subjected to a drying action prior to discharge.) However, the cohesive nature of the heavy phase material may be inconsistent. If a breakdown in the heavy phase dam formed at the heavy phase discharge weir occurs, a "washout" results. A washout is the result of the liquid head moving over the heavy phase discharge weir and, thus, a breakdown of the desired separate discharge of heavy and light phases. Moreover, the operation of a decanter centrifuge is generally required to be steady and continuous, that is, without constant operator assistance. If a washout occurs, substantial modification of the operation of the decanter centrifuge is required in order to rebuild the heavy phase dam at the discharge weir and to again achieve steady state operation. Moreover, in order to avoid a washout, constant supervision of the centrifuge may be required.
Lee U.S Pat. No. 3,795,361 also teaches the operation of a decanter centrifuge in an "above spillover" condition. The Lee decanter centrifuge includes an annular baffle mounted on the screw conveyor. The baffle, which may be made in a number of forms, such as a disc or a cone, extends radially outward from the conveyor hub to a distance where its peripheral edge is in a closely spaced relationship with the inside bowl wall. The outside diameter of the baffle penetrates into the outer, heavy phase layer to form a restricted passageway. The restricted passageway permits the underflow of only heavy phase material at the bowl wall, past the baffle, and into the conical end of the bowl. Thus, the baffle divides the bowl into a cylindrical separating zone, where the centrifugal force separates the heavy phase from the light phase liquid, and a discharge zone, where only heavy phase is present. The Lee decanter centrifuge creates a centrifugal pressure head within the separating zone. This pressure head is the result of the liquid weir being radially inward of the heavy phase discharge weir. This pressure head acts in cooperation with the baffle to provide a supplemental discharge force that assists the screw conveyor in discharging the heavy phase material. This supplemental force created in the separating zone is applied to the separated heavy phase, through the restricted passageway formed by the baffle, and into the discharge zone. The centrifugal pressure head applies a force that assists the conveyor in advancing the heavy phase material up the beach to the discharge ports.
Epper, et. al U.S Pat. No. 4,617,010 shows a decanter centrifuge and/or a nozzle-type centrifuge having a series of projections mounted on the bowl wall along a conical portion thereof so as to create a conveying action in addition to a shearing action on the heavy phase prior to reaching the discharge port. The shearing elements in Epper are formed to assist the discharge of the heavy phase solids up the beach toward the discharge port and, thus, replaces the flights of the conveyor. The Epper shearing elements are also shown in conjunction with a Lee-type baffle. However, the operation of the various Epper decanter centrifuges appears to be in a below spillover condition.
The typical application for a Lee type decanter centrifuge is on heavy phase materials which are considered difficult to convey. The physical characteristics of these difficult-to-convey heavy phase materials, being soft and slimy, are such that the screw conveyor alone cannot normally move them up the beach to the heavy phase discharge ports in a normal below spillover decanter centrifuge type operation. Moreover, these difficult-to-convey materials are contemplated to be of insufficient cohesive nature in order to create a dam at the heavy phase discharge weir for the creation of the Ambler-type operation.
Difficult-to-convey materials are typically found in the operation of a waste water treatment plant. A thickening type operation results in a concentration of the discharged heavy phase material between 3% to 10% solids by weight. As a comparison, a dewatering-type operation produces a heavy phase discharge which has a concentration in excess of 10% solids (by weight), such that the resulting heavy phase may be disposed of by trucking or incineration.
Often within the operation of a decanter centrifuge, chemicals are used to condition feed materials to assist settling and/or coagulation of the solids in the formation of the heavy phase. Such chemicals are typically known as polymers, polyelectrolytes or flocculents. In a dewatering type operation, polymers are almost always required. However, in a thickening type operation, chemicals may or may not be used depending on the type of centrifuge, the nature of the feed material, and the desired heavy phase output concentration. It should be noted that the nature of the heavy phase material varies greatly from application to application due to the specific processes under which the feed material has been placed. Moreover, the application of chemicals to the feed mixture results in a more easily conveyable heavy phase material.